1. Field of the Invention
The present invention relates to a method and a device for controlling the transmitting power of a base station in a mobile communication system based on Code Division Multiple Access (hereinafter abbreviated CDMA).
2. Description of the Related Art
In a CDMA system, transmission is performed by carrying out spread modulation that uses a spread code of high-speed transmission rate after normal information modulation. On the receiving side, normal demodulation is performed after restoring the original information bandwidth by carrying out de-spread modulation that uses the same spread code as the transmitting side. Assignment of an orthogonal spread code sequence to each user allows a plurality of users to communicate simultaneously on the same frequency bandwidth.
In a cellular mobile communication system using CDMA, a plurality of mobile stations share the same frequency band. In communication from a base station to a mobile station, the desired wave signal component and interference wave signal component contained within signals transmitted from the same base station all reach the mobile stations by way of the same propagation path. When a call to mobile station A and a call to mobile station B are made simultaneously from the same base station, for example, mobile station A receives the call to itself as desired wave signal SA and receives the transmission signal to mobile station B as interference wave signal SB. However, these received signal components SA and SB have the same fluctuation characteristics because they have been received by way of the same transmission path from the base station to mobile station A, and as a result, Signal-to-Interference power Ratio (ratio of the received power of the desired wave to the received power of the interference wave hereinbelow abbreviated as xe2x80x9cSIRxe2x80x9d) at the mobile station is constant fixed regardless of the position of the mobile station.
However, when the mobile station receives a desired wave signal from a particular base station X and receives an interference wave signal from another base station Y, the signal component transmitted from base station X and the signal component transmitted from base station Y reach the mobile station by way of different propagation paths. Therefore, the fluctuation characteristics of these signal components also differ, and consequently, a mobile station located near a cell boundary is greatly influenced by the interference wave signal from another base station.
In such cases, degradation of speech quality can be avoided by raising the transmitting power of a base station above the reference power for a mobile station located near a cell boundary. Similarly, degradation in speech quality can also be avoided by increasing the base station transmitting power above the reference power for a mobile station located at a point reached by a plurality of strong multipath signals. On the other hand, the transmitting power of the base station must be reduced below the reference transmitting power for other mobile stations that have good speech quality because the interference wave power received by other mobile stations that are communicating with the base station must be reduced. These factors call for control of the transmitting power of the base station in order to obtain uniform speech quality within a service area.
One example of a method for controlling transmitting power according to the prior art is disclosed in the paper xe2x80x9cOn the System Design Aspects of Code Division Multiple Access (CDMA) Applied to Digital Cellular and Personal Communication Networksxe2x80x9d by A. Salmasi and K. S. Gilhousen and published in IEEE VCT 1991 (pp. 57-62).
FIG. 1 shows a schematic representation of a device for controlling the transmitting power of a base station disclosed in the above-described paper. This device comprises a transmitter, a receiver, and antenna branching filter 108. The transmitter consists of a number N of communication channel transmitters 101 for communication channels 1-N, pilot channel transmitter 102, transmitting power decision unit 106, and amplifier 107.
Transmission signals to mobile station 113 (forward transmission signals) are inputted to communication channel transmitters 101 when the signals are speech signals, and to pilot channel transmitter 102 when the signals are pilot signals. After carrying out information modulation and frequency conversion to the radio frequency bandwidth, each individual channel transmitter 101 and 102 spread-modulates the frequency-converted signals using the spread code corresponding to the object mobile station, and outputs the result. The signal generated by adding the output signals of all channels is then outputted to amplifier 107. The signal amplified by amplifier 107 is then transmitted to mobile station 113 by antenna branching filter 108. Communication channel transmitters 101 set the communication channel transmitting power based on information notified from transmitting power decision unit 106 (to be described hereinbelow).
The receiver consists of amplifier 109, a number N of communication channel receivers 110 for communication channels 1-N, pilot channel receiver 111, and quality judging unit 112.
Amplifier 109 amplifies the spread-modulated radio signal received from mobile station 113 by way of antenna and antenna branching filter 108. Communication channel receiver 110 and pilot channel receiver 111 first subject the radio signal amplified by amplifier 109 to de-spread modulation and extract a radio frequency bandwidth signal, and then frequency-convert the radio frequency signal to a baseband signal, generate an analog signal by demodulation, and output the result. In addition, communication channel receivers 110 each decode quality information attached to radio signals received from mobile station 113 (information indicating the quality of signals transmitted to mobile station 113 from the base station, i.e., SIR as measured at mobile station 113), and output to quality judging unit 112. Quality judging unit 112 judges the quality of signals transmitted from the base station to mobile station 113 based on this quality information and notifies transmitting power decision unit 106. Transmitting power decision unit 106 sets the communication channel transmitting power for the mobile station based on the quality notified from quality judging unit 112 and sets each transmitter 101 to this transmitting power.
In a device for controlling transmitting power of a base station according to this configuration, the transmitting power of a base station is controlled based on the SIR measured at a mobile station. When the SIR measured at the mobile station is lower than a reference value, a request signal to increase the transmitting power is transmitted to the base station. When the measured SIR is greater than the reference value, a request signal to decrease the transmitting power is transmitted to the base station. The base station receives these request signals and either increases or decreases the transmitting power by a prescribed control step power, a small value such as 0.5 dB being used as the control step power. Control is limited to a range of, for example, xc2x16 dB with respect to the reference power.
In a case in which the total transmitting power of the base station increases and approaches a predetermined maximum transmitting power in the device for controlling transmitting power of a base station of the prior art described hereinabove, the base station is unable to increase the power in accordance with the demand of a mobile station despite the transmission of request signals to increase transmitting power from a mobile station because the total transmitting power would then exceed the maximum transmitting power. In the event of insufficient transmitting power in such a case, there is the problem that the waveform of the transmitting signal of the base station is distorted, thereby preventing correct demodulation of the signal at the mobile station and giving rise to degradation of speech quality. Such cases of insufficient base station transmitting power tend to occur at base stations having, for example, increased traffic.
It is an object of the present invention is to provide a method and device for controlling the transmitting power of a base station in a CDMA cellular system that can reduce degradation in forward speech quality in cases in which the total base station transmitting power increases and the base station is unable to transmit the power desired by mobile stations.
To achieve the above-described object, in the method of controlling the transmitting power of a base station in a CDMA mobile communication system according to the present invention, a desired transmitting power Pa(i) of a transmitting power control object channel i which is one channel selected for transmitting power control from among a plurality of communication channels, is calculated. This desired transmitting power Pa(i) is then added to the transmitting powers Ptx(j) that are currently set to communication channels j, which are communication channels other than this transmitting power control object channel i, and this sum is taken as the total base station transmitting power Ptotal.
The total base station transmitting power Ptotal is then compared with a predetermined maximum total transmitting power Pmax, and when the total base station transmitting power Ptotal is greater than the maximum total transmitting power Pmax, the total base station transmitting power Ptotal is divided by the maximum total transmitting power Pmax to calculate the proportion of insufficient transmitting power and find a power reduction rate Dt.
A value obtained by using power reduction rate Dt to reduce the desired transmitting power Pa(i) is then set as the new transmitting power of transmitting power control object channel i, and a value obtained by using power reduction rate Dt to reduce each transmitting power Ptx(j) currently set to each current communication channel j is set as the new transmitting power of each of communication channels j.
The present invention allows the total base station transmitting power Ptotal to be controlled so as not to exceed the maximum total transmitting power Pmax by reducing the transmitting power of each communication channel by this power reduction rate Dt.
In addition, in the another method of controlling transmitting power of a base station in a CDMA mobile communication system of the present invention, one channel is selected as the transmitting power control object channel i from among a plurality of communication channels at regular intervals; and, in a transmitting power control process that updates the setting of the transmitting power of each communication channel such that the transmitting power control object channel i satisfies a prescribed speech quality, stores as the previous reduction rate Dtxe2x88x921 the power reduction rate of the total transmitting power that was used in the preceding processing.
For communication channels j other than the transmitting power control object channel i, values obtained by multiplying transmitting powers Ptx(j) currently set to each of communication channels as the result of the previous transmitting power control process by the previous reduction rate Dtxe2x88x921 is output as the provisional transmitting powers of these communication channels j.
For the transmitting power control object channel i, when the value obtained by multiplying the desired transmitting power Pa(i) by a prescribed function of the previous reduction rate Dtxe2x88x921 is equal to or less than a specified maximum transmitting power per channel Pmaxxe2x88x921ch, that power is made to be the provisional transmitting power of that channel i; when the value obtained by multiplying the desired transmitting power Pa(i) by a prescribed function of the previous reduction rate Dtxe2x88x921 is greater than the specified maximum transmitting power per channel Pmaxxe2x88x921ch, a value obtained by dividing the maximum transmitting power per channel Pmaxxe2x88x921ch by the prescribed function is made to be the provisional transmitting power of that channel i.
The total base station transmitting power Ptotal is then calculated by adding the provisional transmitting power of each communication channel, and the power reduction rate Dt, which is the proportion by which the total transmitting power of the base station is reduced, is calculated by dividing this total base station transmitting power Ptotal by the specified maximum total transmitting power Pmax.
When the total base station transmitting power Ptotal is greater than the maximum total transmitting power Pmax, the provisional transmitting power of each communication channel is divided by power reduction rate Dt, and this divided transmitting power is set as the new transmitting power of each communication channel. When the total base station transmitting power Ptotal is equal to or less than the maximum total transmitting power Pmax, each provisional transmitting power is divided by the previous reduction rate Dtxe2x88x921, and this divided transmitting power is set to each communication channel as the new transmitting power of each communication channel.
In the method of controlling the transmitting power of a base station of the present invention, the transmitting power of each individual communication channel that has been reduced by the previous transmitting power control process are returned to the transmitting power before reduction by multiplying by a prescribed function of the previous reduction rate Dtxe2x88x921 for transmitting power control object channel i and multiplying by previous reduction rate Dtxe2x88x921 for communication channels j; then these transmitting powers are made the provisional transmitting powers; and these provisional transmitting powers are used to effect control of transmitting power.
Accordingly, control is effected whereby the transmitting power of each communication channel is returned to its original value when the total base station transmitting power Ptotal no longer exceeds the maximum total transmitting power Pmax.
In addition, according to another method of controlling the transmitting power of a base station in a CDMA mobile communication system of the present invention, the prescribed function of the previous reduction rate Dtxe2x88x921 is a function by which the previous reduction rate Dtxe2x88x921 is raised to the xcex1 power.
In the another method of controlling the transmitting power of a base station in a CDMA mobile communication system according to the present invention, the desired transmitting power Pa(i) of transmitting power control object channel i, is calculated which is one channel selected for transmitting power control from among a plurality of communication channels.
The total base station transmitting power Ptotal is then calculated by adding the desired transmitting power Pa(i) and transmitting power Ptx(j) currently set to communication channels j, which are communication channels other than transmitting power control object channel i.
Power conversion rate Ct is then found by dividing the total base station transmitting power Ptotal by the maximum total transmitting power Pmax.
A value obtained by using power conversion rate Ct to convert desired transmitting power Pa(i) is then set as the new transmitting power of transmitting power control object channel i, and values obtained by using power conversion rate Ct to convert the transmitting power Ptx(j) currently set to each of communication channels j are set as the new transmitting powers of communication channels j.
In this method of controlling transmitting power of a base station of the present invention, the value of the total base station transmitting power Ptotal is always equal to the maximum total transmitting power Pmax by multiplying the set transmitting power by the power conversion rate.
Accordingly, the amount of interference from the base station no longer depends on the volume of traffic because the total base station transmitting power Ptotal is always constant, and the design of a base station device is accordingly simplified.
As described hereinabove, the present invention has the effect of preventing the total transmitting power of a base station from exceeding the maximum transmitting power by calculating the total transmitting power when the desired transmitting power of each channel is being transmitted and then reducing the transmitting power when this total transmitting power is equal to or greater than the maximum total transmitting power of the base station.
The above and other objects, features, and advantages of the present invention will become apparent from the following description with references to the accompanying drawings which illustrate examples of the present invention.